At present, the most common technique for defining an alphanumeric and symbol character set in a raster display system is to define each character is a read-rastered dot matrix format which explicitly defines the ON and OFF pels for the character and maps one-to-one to the display surface (such as a CRT screen) in the region of the surface where the character is to be positioned; see, for example, page 115 of the book "Fundamentals of Interactive Computer Graphics" by Foley and Van Dam, published 1982 by the Addison-Wesley Publishing Company. The advantage of characters defined in dot matrix format is that they do not require vector-to-raster conversion and are therefore speedily made available to the display device when specified for display. However, the dot matrix format is highly inefficient as regards storage space since each bit of the matrix is stored irrespective of whether this represents a visible part of the character (e.g., an ON pel for a light on dark image) or a part of the background (an OFF pel). For example, for characters defined in a 14 by 20 matrix, at least 280 bits are required to define each character regardless of the complexity of the character.
It is therefore an object of the present invention to provide an improved method of storing characters which is more economical of storage space than the prior art referred to above, but which does not achieve this at the expense of greatly increased processing complexity when the characters are specified for display.